1. Field of the Invention
The present invention relates to grids for linear beam RF (radio frequency) vacuum tube devices having an electron emitting cathode and an RF-modulated grid closely spaced thereform.
2. Description of the Related Art
It is well known in the art to utilize a vacuum electron (VED) device, such as a klystron or traveling wave tube amplifier, to generate or amplify high frequency RF energy. Such devices generally include an electron-emitting cathode and an anode spaced therefrom. In the case of IOTs (Inductive Output Tubes) and similar devices, a grid is also included, positioned in an inter-electrode region between the cathode and the anode. Grid-to-cathode spacing is critical and is directly related to the performance and longevity of the linear beam device. The material of the grid is typically pyrolytic graphite (PG), selected for its excellent thermal properties.
The grid is typically formed by a hydrocarbon gas deposition process over a mandrel into a blank cylinder or “cup,” designated 10, in FIG. 1a. The closed top end 12 of the cup 10 is shaped to include a dome 14. Cup 10 is then cut, along the dashed line L, with the intention of forming a flange 16 (FIG. 1b) that should be concentric with the dome 14, for purposes of proper alignment during assembly and operation of the vacuum electron device (VED). Dome 14 is then laser cut (FIG. 1c) to provide the characteristic grid features. A side elevational view of the finished grid is provided in FIG. 1d. 
Deposition of the PG to form the cup 10 involves a high-temperature CVD (chemical vapor deposition) process that breaks down the hydrocarbon gas, thereby depositing a pyrolytic graphite coating onto the precision CNC-machined mandrel (not shown). The deposited PG coating is allowed to grow in thickness so that it becomes self-sufficient and subsequently releases from the mandrel as the film and mandrel are cooled from high temperatures. Since the hydrocarbon gas deposits carbonaceous growths on all surfaces exposed to the gas flow in the working area of the CVD furnace hot zone, the contiguity of the film can be stressed at discontinuities or sharp bends. These discontinuities then become stress concentrators. As the film cools down, the desired portion of the film can interact with these stress concentrators and thereby develop significant internal residual thermal stresses, regardless of consideration of mandrel material. These film stresses result from the severe anisotropy of the in-plane and out-of-plane thermal expansion coefficients and the restriction that the graphitic layers fully remain contiguous across an imaginary vector normal to the film growth surface. This contiguity restriction results from the inability of the carbon atoms to diffuse and realign in any meaningful timescale at the deposition temperature (nominally about 1700-2200° C.) and below. When the differences in coefficients of thermal expansion between the film and the underlying mandrel are taken into effect, the residual thermal stresses are amplified. As a result, discontinuities often act as crack initiation sites. The cracks can propagate a sufficient distance in the material to destroy the desired product, reducing cup yield to as low as 30% during manufacture of the cup 10, before the laser cutting stage of FIG. 1c is even reached. Even when the residual thermal stresses are not immediately manifested, they render the product particularly fragile and subject to warping, or “potato-chipping,” at handling, assembly, and during normal operation.